Producing synthetic resin foams by reacting polyols with polyisocyanate compounds in the presence of catalysts and blowing agents has been widely practiced. Examples of synthetic resin foams thus obtained are polyurethane, polyisocyanurate, etc.
Trichlorofluoromethane (CFC-11) has been mainly used as an organic blowing agent in the production of synthetic resin foams such as the aforementioned polyurethane foams.
In recent years, it has been pointed out that some types of chlorofluorocarbons, once released in the atmosphere, deplete the ozone layer of the stratosphere, and as a result, the ecosystem including humans may be severely adversely affected. Therefore, the use of chlorofluorocarbons posing a high risk of ozone layer depletion is restricted under international agreements, including the use of CFC-11. Accordingly, the development of novel blowing agents that are free from or have a small risk of ozone layer depletion is required.
As a chlorofluorocarbon that has a low impact on the ozone layer, 1,1-dichloro-1-fluoroethane (HCFC-141b) is substituted for CFC-11.
However, this substance incorporates chlorine atoms in its molecular structure, and thus still poses some risk of depleting the ozone layer.
Japanese Unexamined Patent Publication Nos. 29440/1990 and 235982/1990 disclose processes for producing foams using fluorinated hydrocarbons that do not contain chlorine and have no risk of ozone layer depletion. Moreover, Japanese Unexamined Patent Publication No. 235982/1990 discloses the use of 1,1,1,3,3-pentafluorobutane (HFC-365mfc) as a blowing agent for plastic foam production. U.S. Pat. No. 6,380,275 discloses a blowing agent containing HFC365 and a low-boiling HFC.
HFC-365mfc is a hydrogen-containing fluorinated hydrocarbon having a boiling point of 40° C.; therefore, it is considered to have no risk of ozone layer depletion. Moreover, since HFC-365mfc has a boiling point comparable to that of HCFC-141b, HFC-356mfc is drawing attention as a leading candidate to substitute for HCFC-141b as a blowing agent. Moreover, it has a thermal conductivity of 15.5 mW/mK (transient hot-wire method, 50° C., 1 atmospheric pressure), having the qualities necessary for a blowing agent.
HFC-365mfc, although considered to have no flash point under the Japanese Fire Service Law, does have a flammability range (mixing ratio with air: 3.6 to 13.3 vol %). Therefore, it may turn flammable under some circumstances. Since the important point of HFC-containing blowing agents is their nonflammability, the possibility of ignition is a significant barrier to the use of HFC-365mfc.
Thus, to effectively use HFC-365mfc as a substitute for HCFC-141b, the development of techniques to control its flammability to render HFC-365mfc substantially nonflammable is necessary.
U.S. Pat. No. 6,380,275 discloses blowing agents containing HFC365 and low-boiling HFCs. These low-boiling blowing agents contain flammable substances (for example, HFC152a, HFC32, dimethyl ether, propane, etc.), and the purpose of using low-boiling HFCs is to enhance foaming and the properties of foamed articles, not aiming to render HFC365mfc itself nonflammable.
The present invention was accomplished in view of the prior art problems. One of the primary objects of the invention is to provide a blowing agent that, when mixed with a polyol to form a premix, renders the premix substantially nonflammable despite blend of HFC-365mfc.
Another primary object of the invention is to provide a process for producing a synthetic resin foam having excellent thermal insulation properties and outstanding mechanical strength.